A digital display device comprises an array of pixels which can each be in an ‘on’ state or an ‘off’ state. A process for generating an image for display may comprise determining which pixels should be in an ‘on’ or ‘off’ state at any one time in order to define a required image on the display device. Typically, a data set may be generated that is useable by the display device to control the state of any one or more of the pixels thereby to generate a desired image.
For the purposes of the present patent application, the process of receiving input data defining a graphical element to be displayed and of defining the state of one or more pixels in a digital display device in order to display that graphical element will be referred to as ‘plotting’. Devices implementing such a plotting process will be referred to as ‘plotters’ and the meaning of the verb ‘to plot’, and its various tenses, are be interpreted accordingly.
Known image generators for cathode ray tube (CRT) type displays may be configured to operate in either a raster mode or a cursive mode. In a raster mode, an image is generated by controlling the state of the pixels in sequence, usually in horizontal lines, starting each display frame at the pixel at the top left of the display and ending at the pixel at the bottom right of the display. In a cursive mode, an image is defined in terms of geometric graphical elements such as a lines or curves to be displayed at defined positions within an array of pixels. Input image data are therefore ‘plotted’ to ‘draw’ the defined graphical elements on the display device starting at a defined pixel and passing through other defined pixels. Raster mode is typically preferred when the state of most or all of the pixels of a display will be changing frequently, for example if a video image is displayed. Cursive mode on the other hand is preferred when symbols are displayed which occupy only a small portion of the image area of the display device.
Digital display devices, such as digital micro-mirror devices or Rapid Update digital display devices, offer many advantages over CRT displays because they are typically lighter, smaller and can operate using low voltages. However, the process of plotting an image defined in terms of geometrical symbols for such a display device may incur a delay and it is desirable to provide a process for generating and displaying images so defined with reduced latency.
Typically a plotter will plot one or more images into an image buffer during a refresh period of a corresponding display device and when all of the elements of the image or images have been assembled in the buffer, the resultant image data may be transferred to the digital display device for display. An example of a typical display device and image generating technique is described with reference to FIGS. 7 and 8. At the start of a cycle, or refresh period RP, an image generator 100 generates imagery inputs 1 to 4 representing portions of an image which are transferred sequentially with respect to time ‘t’ for representation in an image buffer 102. The image buffer adds together the imagery inputs such that at a time t=4 it contains inputs 1, 2, 3, 4. The content of the image buffer is shown diagrammatically in FIG. 8 for each of times t=1, 2, 3 and 4. As shown the image contained in the buffer at time t=4 is a square and the imagery inputs define the sides of the square which are generated one after another. Once the full image (i.e. the square) is received, it is transferred to a digital display device 104 for display. The time taken to transfer the full image to the display is the transport period TP and is represented by the downwardly sloping arrow between the buffer 102 and display device 104 in FIG. 7.
It will be seen there is a delay between the start of a cycle at a time t=0 and the reproduction of the full image by the device 104 which is equal to the refresh period plus the transport period.